Perhaps they should have called this the Curve Ball tour. Tom Morello, filling in for Steven Van Zandt during the E Street Band's ongoing concerts through Australia, says Bruce Springsteen tinkers with the setlist until the absolute last moment.

That's made for some of the most diverse, deep cut-filled performances of Springsteen's storied live career, as fans have thrilled to rarely heard tracks like "Be True," "High Hopes" and "Bishop Danced," along with new songs from the well-received 2012 effort 'Wrecking Ball.'

But their guest guitarist certainly has had to stay on his toes.

"Every night, 90 minutes 'til soundcheck, Bruce will text me with seven or eight songs we've never played before," the Rage Against the Machine guitarist tells Rolling Stone. "And then during the show, he'll call up songs we've never even discussed – some I've never even heard!

Morello appeared on two tracks from 'Wrecking Ball' before taking over while Van Zandt films his television show 'Lilyhammer.' Morello says he started out by learning 50 Springsteen songs over three months, but even that hasn't been enough.

Not that he's complaining.

"It's been great," Morello adds. "It's been really an honor being onstage with one of my favorite bands – one of the greatest live bands of all time. I've been to a lot of Bruce Springsteen shows, but I've never been to four consecutive ones. And every show isn't just a different show – it's a completely different experience."

Van Zandt has been with the E Street Band since 1975, leaving once before between 1984-95 for a solo career. He's set to rejoin Springsteen and Co. for the European leg of this tour, beginning on April 29 in Oslo.